The present invention relates to a constant-current circuit, and in particular to a constant-current circuit which is employed for a logic circuit of FET formed on a substrate, such as a GaAs substrate, and which is little affected by differences in the characteristics of a device and by power voltage fluctuations.
Since an MESFET (Metal Semiconductor Field Effect Transistor) formed on a GaAs semiconductor substrate has a faster operation speed, a higher frequency characteristic and a lower power consumption than an MOS transistor employing a silicon semiconductor substrate, attention is focused on the MESFET as a device constituting an LSI to be used for fast signal processing in a communication system. A representative logic circuit is an SCFL (Source Coupled FET Logic) wherein FET source terminals are connected in common and a constant-current source is connected between the commonly connected source terminals and a lower power voltage side, a load being connected between a drain terminal and a power voltage terminal. This logic circuit is similar to an ECL (Emitter Coupled Logic) circuit which employs a bipolar transistor formed on a silicon substrate, and a combination of the two logic circuits is frequently employed.
Recently, there have been instances where an SCFL circuit has been employed together with a CMOS circuit using silicon.
FIG. 4 is a circuit diagram illustrating a conventional constant-current circuit. This example shows a constant-current circuit constituting a constant-current source for the above SCFL circuit. In the SCFL circuit, source terminals of transistors Q.sub.10 and Q.sub.12 are connected in common, loads R.sub.10 and R.sub.12 are connected between the their drains and power supply voltage V.sub.DD, and a transistor Q.sub.14 is connected as a constant-current device between a ground power supply voltage and the common source terminal. Input signals IN and /IN having opposite phases are transmitted to the gates of the transistors Q.sub.10 and Q.sub.12, and in accordance with the level, H or L, of the input signals, output signals are generated at output OUT and /OUT. When current I.sub.B is set to a constant-current, a level lower by R.sub.10 .times.I.sub.B than power voltage V.sub.DD can be set as a fixed level L for the output signal.
In the prior art, the constant-current circuit is constituted by the transistor Q.sub.14 and resistors R.sub.1 and R.sub.2 connected between the power voltage source and the ground. A bias voltage V.sub.B divided by the resistors R.sub.1 and R.sub.2 is applied to the gate terminal of the transistor Q.sub.14. When the bias voltage V.sub.B has a constant potential, the voltage between the gate and the source of the transistor Q.sub.14 is constant and current I.sub.B serves as a constant-current
In the constant-current circuit shown in FIG. 4, however, a constant current I.sub.B can not be produced because of variations in the power supply voltage V.sub.DD, the characteristic differences of the resistors and the threshold voltages for the transistors, and characteristic differences which accompany temperature changes.
FIG. 5 is a graph showing the relationship between a current I.sub.o flowing in the circuit comprising the resistors R.sub.1 and R.sub.2 and the bias voltage source V.sub.B. Since the bias voltage V.sub.B is determined from a product of the resistance R.sub.2 and the current I.sub.o, the relational equation is EQU I.sub.o =V.sub.B /R.sub.2.
And since a differential voltage between the power supply voltage V.sub.DD and the bias voltage V.sub.B is applied to the resistor R.sub.1, and the current I.sub.o flows across it, the load characteristic is EQU I.sub.o =-V.sub.B /R.sub.1 +V.sub.DD /R.sub.1.
The above relationship is shown in FIG. 5. The solid line represents the characteristics of the resistor R.sub.2, and the broken lines and the chained lines represent the characteristics of the resistor R.sub.1. The intersections of the several characteristic lines are operation points.
As the power supply voltage V.sub.DD changes, the load characteristic is changed to the right or to the left, as is indicted by the broken lines. In addition, the resistance of the resistor R.sub.1 is varied due to manufacturing variances and temperature changes, and the load characteristic is changed as is indicated by the chained line. As a result, the operation points are also changed, and there is a great voltage change .DELTA.V.sub.B in the bias voltage V.sub.B. The fluctuation of the bias voltage V.sub.B changes the voltage between the gate and the source of the transistor Q.sub.14 and induces the fluctuation of the current I.sub.B of the constant-current source.
Further, when the threshold voltage of the transistor is changed due to a manufacturing variance, even though the bias voltage V.sub.B is constant, the drain current I.sub.B flowing through the transistor Q.sub.14 is changed.
Generally, an MESFET using a GaAs substrate is so designed that a Schottky diode comprising a metal gate electrode is formed on an active layer deposited on the surface of the GaAs substrate, and employs, for its basic operation, the control of a depletion region in the active layer by controlling a gate voltage applied to the gate electrode. In order to provide a certain constant thickness for the active layer under the gate electrode, a process for forming a groove is performed in an area in which the gate electrode is to be formed. Thus, variations in the threshold voltage of a transistor, accompanied by manufacturing variances, can not be avoided. In addition, the characteristics of a resistor element formed on the GaAs substrate differs depending on the quantity and the depth of an ion implantation. It is also well known that temperature changes can delicately vary the characteristics of the resistor element.
As is described above, changes in the power voltage and differences in the characteristics of an element are problems that can not be avoided, and the formation of a constant-current source is desired which operates under such a condition.